Telomere sequences and their regulation have been implicated in- both cellular aging and the development of cancer; however, it is clear that these relationships are not as simple as first thought. Drosophila is a valuable model for the study of telomeres because this organism maintains its telomeres by a variation of the mechanism used by most other species. Drosophila telomeres are composed of multiple repeats of the telomere specific non-LTR retrotransposons, HeT-A and TART, rather than the simple repeats generated by telomerase. Comparison of similarities and differences between these two mechanisms offers a powerful approach for determining the essential features of telomere activity, a long term goal of our work. A primary goal of these experiments is to understand the regulation of these retrotransposons. One line of experiments will focus on the recently identified HeT-A promoter. This promoter is interesting not only for its role in telomere maintenance, but because it is the first heterochromatic promoter studied at the molecular level. Additionally, it is an unexpected intermediate between the typical promoters for non-LTR and LTR elements, a finding with evolutionary implications vis a vis retroviruses. Studies in cultured cells have detected several functional elements within the promoter sequence. This work will be extended to clearly define each element, to test the promoter by using reporter transgenes in intact flies, to identify any tissue-specific elements in the promoter, and to explore interactions with both heterochromatic and euchromatic chromosomal environments. TART yields both sense and antisense transcripts. The antisense promoter transcript was identified; it and the sense promoter will be defined. Both are heterochromatic and will be characterized as was the HeT-A promoter. HeT-A expression is dramatically increased in older flies and some cell cultures. We hypothesize that this is due to age-related changes in heterochromatin and will test this and the effects of factors affecting position effect variegation on the expression of the HeT-A and TART promoters. The origin of a novel related reverse transcriptase mRNA will be determined. Evidence suggests that this RNA is either a processed product of TART or the mRNA from a "free-standing" cellular gene. This mRNA is interesting because of a possible evolutionary relationship to the catalytic subunit of telomerase. The phylogenetic study of the transposon mechanism for telomere maintenance will be extended to include insects outside the genus Drosophila to broaden the view of its essential features of this mechanism.